1. Field of the Invention
The subject matter of this disclosure relates to the electrical heating of metal cylinders to form a seam bond and applying a means of cooling to the metal cylinders corresponding at least in part to U.S. Classification 261/61.7 and IPC8 B23K9/02.
2. Description of Related Art
Industrial pipe systems involve multiple pipe configurations, different diameters and pipe wall thicknesses often joined to one of numerous connection elements such as flanges, elbows, T junctions. Engineers draft spool drawings as representations of a pipe section that needs to be created. These drawings detail the angles, fitting sizes, and other specifications needed to create the desired pipe structures (fabricated pieces). The pipe assembly process generally begins with preparing the segments. Bevels are created on the pipe ends to lay down the multiple weld passes, and other pipe end or surface preparations are performed. Next the spool components are generally tack welded together to align the pipe sections for multi-pass welding.
Multi-pass welding is traditionally performed manually by specialized and highly skilled welders. However, various forms of automation exist. A common automation is flat-position groove welding. In flat-position groove welding, two or more pipe sections are set horizontally on a support structure. The support structure rotates the pipes for welding. A fixed welding torch is aligned with the pipe junction and the pipes sections are welded together while the pipes are rotating. Automated welding can include a fixed or stationary pipe, with the automated welding torch rotating around the pipe as it welds. Other forms of semi-automation include manual weld first passes with orbital GTAW or orbital FCAW for subsequent weld passes. Finally, multi-pass welding may be performed by robotic arms programmed to apply welds according to the specifications of the spools.
It is common that welding codes and or procedures limit the process to a maximum interpass temperature during the entire multi pass weld. The process of multi-pass arc welding generally involves the steps of a) creating a first weld seal (root pass) of two sections of pipe, b) allowing the weld to cool, c) performing a subsequent weld over the previous weld, and d) repeating steps b) and c) until the piping sections are fully welded together across the thickness of the pipe wall, including weld reinforcement. Many codes or weld procedures require the weld area on the inside of the pipe to be free from atmospheric contaminants such as oxygen and sometimes nitrogen. In order to accomplish this, dams are inserted into the two sections of pipe while performing welding. These dams form a contained interior volume of piping with the weld seam generally in the center. In many circumstances, both water and oxygen should be minimized or eliminated while the weld is being made and during the post welding cool down period. Consequently, during arc welding of thick walled pipe sections together, an inert gas is continually flushed through the contained interior volume to prevent oxidation of the weld site and to evaporate and remove moisture from the weld site.
A major materials issue with multi-pass welding of pipes is the structural integrality of the resulting weld. It is critical in e.g. nuclear reactors, that pipes handling reactor coolant not fail due to rupture. A stress fracture of key pipes in a refinery could result in catastrophic failure causing great damage and endangering many lives. It is thus essential that these welded structures adequately withstand the extreme conditions to which they are exposed.
A variety of standards exist which quantify various material requirements for multi-pass welded pipes. See, e.g., Process Piping: The Complete Guide to ASME B31.3. Third Edition. Charles Becht IV. ASME Press, Three Park Avenue, New York, N.Y. 10016-5990. 2009; ISBN-13: 978-0-7918-0286-1. A key control parameter in producing multi-pass welded pipes is the “interpass temperature” parameter. There are empirically defined minimum and maximum interpass temperatures depending on such factors as the type of metal alloy making up the pipe sections. These temperatures define welding process conditions that produce pipe welds with acceptable material properties. In particular, before a subsequent welding pass, the weld site temperature should be at or below the maximum interpass temperature. In practice this requires waiting for the prior weld's temperature to drop to at least the interpass maximum temperature. The interval time between weld passes in the current practice will vary according to wall thickness and maximum interpass temperature but can range from a few minutes to an hour or more. This slows down the welding process and causes undesirable idle time for highly skilled specialty welders. There is also an ongoing risk of overheating the weld zone thereby causing structural flaws in the piping produced. A severe example of such structural flaws from overheating of a metal during welding is warping and distortion of the physical shape of the material.
It is therefore desirable to effect control over the temperature of weld sites in multi-pass welding to reduce or eliminate the down time between welding passes. The art has not effectively addressed this problem. Solutions to interpass temperature control generally relate to accelerated cooling between passes. These prior art operate by application of air and/or water for convective heat transfer from the weld. Use of air exposes the weld to oxygen and is thus contraindicated for the pipe welding of this disclosure. Water cooling potentially may be used, but this requires specially adapted equipment. Exposing water to the weld site during weld processing is also undesirable because the water has to be removed after welding, the water can pose safety hazards including electrical and slip and fall, this can also lead to oxidation on the inside of the pipe.
U.S. Pat. No. 4,152,568 describes a process of coolant circulation within a pipe to accelerate the cooling rate of welds. The coolant is water, liquid nitrogen or dry ice. Liquid nitrogen is preferred for cooling from the maximum interpass temperature to 800 degrees C. U.S. Pat. No. 4,152,568 does not address multi-pass welding where three or more welds are applied in series. U.S. Pat. No. 4,152,568 does not describe the control of the maximum temperature reached by the weld site. Finally, U.S. Pat. No. 4,152,568 still requires specially adapted equipment to carry out the described accelerated interpass cooling method. This method does not address the potential for metallurgical changes in the base material as a result of deep cryogenic treatment of the weld zone and other areas where the liquid nitrogen comes in contact with the pipe. This cryogenic treatment can be advantageous by increasing wear resistance in some materials but may also be disadvantageous to some materials by possibly decreasing tensile strength and or other mechanical properties. The limitations and effects are currently being researched.